In dentistry, practitioners use a variety of restorative materials, for example to create crowns, veneers, direct fillings, inlays, onlays and splints. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a resin matrix. These materials may be dispersion reinforced, particulate reinforced, or hybrid composites.
Dispersion reinforced composites include a reinforcing filler having a mean particle size of about 0.05 μm or less, with a filler loading of about 30%-45% by volume. However, loading of the dispersion-reinforcing filler into the resin is limited by the ability of the resin to wet the filler due to the small particle size and high surface area of this type of reinforcing filler. Consequently, the filler loading is limited to about 45% by volume. Due to the low loading, the filler particles are not substantially in contact with one another. Thus, the primary reinforcing mechanism of such dispersion-reinforced composites is by dislocation of flaws in the matrix around the filler. In dispersion-reinforced materials, the strength of the resin matrix contributes significantly to the total strength of the composite. In dentistry, dispersion reinforced composite resins or microfills are typically used for cosmetic restorations due to their ability to retain surface luster. Typically, these microfill resins use free radical-polymerizable resins such as methacrylate monomers, which, after polymerization, are much weaker than the dispersed filler. Despite the dispersion reinforcement, microfill resins are structurally weak, limiting their use to low stress restorations.
Microfills generally use prepolymerized filler material for dispersion reinforcement, as described in U.S. Pat. Nos. 4,389,497, 4,781,940 and 6,020,395. Generally, prepolymerized filler is produced by mixing finely divided silica with a polymerizable monomer, heat polymerizing the mixture in bulk, and pulverizing or grounding the mixture down to the desired agglomerate size to give a filler material comprising splintered polymerized particles. Polymerized particle sizes are large, generally greater than 1 μm in diameter, allowing for better reinforcement but a less stable, less glossy surface. The polymerized particles or prepolymerized filler material is then mixed with a polymerizable monomer, typically an acrylate or methacrylate-based resin, and an additional filler material, such as colloidal or fumed silica, to form the final microfill dental composite.
Particulate reinforced composites typically include a reinforcing filler having an average particle size greater than about 0.6 μm and a filler loading of about 60% by volume. At these high filler loadings, the filler particles begin to contact one another and contribute substantially to the reinforcing mechanism due to the interaction of the particles with one another and to interruption of flaws by the particles themselves. These particulate reinforced composite resins are stronger than microfill resins. As with the dispersion-reinforced composites, the resin matrix typically includes methacrylate monomers. However, the filler in particulate reinforced composites has a greater impact on the total strength of the composite, such that the particulate reinforced composites have been used for stress bearing restorations.
Another class of dental composites, known as hybrid composites, include the features and advantages of dispersion reinforcement and those of particulate reinforcement. Hybrid composite resins contain fillers having an average particle size of 0.6 μm or greater with a microfiller having an average particle size of about 0.05 μm or less. HERCULITE® XRV (Kerr Corp.) is one such example. HERCULITE® has been considered by many as an industry standard for hybrid composites. It has an average particle size of 0.84 μm and a filler loading of 57.5% by volume. The filler is produced by a wet milling process that produces fine particles that are substantially contaminant free. About 10% of this filler exceeds 1.50 μm in average particle size. In clinical use, the surface of HERCULITE® turns to a semi-glossy matte finish over time. Because of this, the restoration may become distinguishable from normal tooth structure when dry, which is not desirable for a cosmetic restoration.
Various methods of forming submicron particles, such as precipitation or sol gel methods, are available to produce particulate reinforcing fillers for hybrid composites. However, these methods do not restrict the particle size to at or below the wavelength of light (about 0.5 μm) to produce a stable glossy surface. U.S. Pat. No. 6,121,344, which is incorporated by reference herein in its entirety, describes a resin-containing dental composite including a structural filler of ground particles having an average particle size of between about 0.05 μm and about 0.5 μm that has the high strength required for load-bearing restorations. Because the structural filler particles are ground, the particles are nonspherical, providing increased adhesion of the resin to the structural filler, thereby further enhancing the overall strength of the composite. Through the use of ground structural filler particles having an average particle size less than the wavelength of light, the dental composite exhibits the luster and translucency required for cosmetic restorations.
In U.S. Pat. No. 6,121,344, fumed silica having an average particle size less than about 0.05 μm are added, preferably between about 1% by weight and about 15% by weight of the composite. The microfill particles contribute to dispersion reinforcement, fill the interstices between the larger structural filler particles reducing occluded volume, and provide a large surface area to be wetted by the resin to increase strength. The fumed silica microfill particles also contribute to the flow properties of the uncured resin. Fumed silica is produced by hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. During this process, silicon dioxide molecules condense to form particles of size usually less than 50 nm. The particles then attach to each other and sinter together. Due to the nature of the flame process, a three-dimensional chain aggregate with a length of 200-300 nm forms. Further mechanical entanglement occurs upon cooling to give agglomerates. Attractive interactions between surface silanol groups of the particles give thixotropic properties to liquids in which these fumed silicas are suspended. The fumed silicas are hydrophobically treated to make it compatible with resins employed, however, treatment is usually not complete and residual unreacted silanol groups typically remain, resulting in substantial interactions of these groups with other reactive groups in the composite. The particle-particle interaction prevents homogenous dispersion of the microfiller in the resin matrix and increases the viscosity of the suspension, which correspondingly decreases the workability of the composite paste. This places a limitation on the practical filler loading in fumed silica microfilled restorative composites. A high filler loading is desirable in dental restorations because the high loading provides a paste with improved handling properties over a paste with low filler loading. Moreover, higher loading gives a composite that experiences lower shrinkage upon curing and has a coefficient of thermal expansion better matching that of a natural tooth.
Resin shrinkage upon polymerization, however, is a problem that has faced composites of the prior art incorporating dispersion reinforced, particulate reinforced and hybrid filler materials. The resin matrix shrinks upon polymerization during the curing process. Polymerization shrinkage, both axial and volumetric, generally results from the conversion of the carbon-carbon double bonds of low molecular weight monomers in the polymeric composite to corresponding carbon-carbon single bonds of crosslinked polymers during the curing reaction. Such shrinkage tends to cause gap formation between the restorative composite and the tooth, leading to microleakage, secondary caries and decreased longevity of the repair.
There is thus a need to develop a hybrid dental restorative composite that has a physical make-up to afford high filler loading, appropriate viscosity for good workability of the composite paste, and lower shrinkage during polymerization.